1. Field of the Invention
The present invention relates generally to positioning systems, and more particularly to methods for using such systems to determine relative differential positioning for transportation applications.
2. Related Art
As is well known in the relevant art(s), the Department of Defense's Global Positioning Satellite (GPS) constellation operationally consists of twenty-four satellites that provide global coverage for determining the geographic position of a user equipped with any of a variety of commercially-available receivers. GPS receivers are capable of receiving the L-band radio signals emitted from the satellites in the constellation whose orbits have an altitude of approximately 12,660 miles above the Earth. For any given signal reading, at least four satellites are required to compute the three dimensions of position (X, Y, and Z or latitude, longitude and altitude, respectively) and time.
More specifically, GPS receivers receive transmissions of at least four satellites and combine the information with information in an electronic almanac, so that it can mathematically determine the receiver's position on Earth in a well-known manner. The basic information a GPS receiver provides is the latitude, longitude and altitude, or some similar measurement, of its current position. Most receivers then combine this data with other information, such as maps, to make the receiver more useable (i.e., more “user friendly”).
Aside from the recreational uses that automobile drivers, boaters, hikers, etc. can make of GPS receivers (an aside from GPS' military applications), there a several large-scale, commercial uses of GPS receiver systems.
For example, the pressure to increase the performance of modern rail (i.e., train) systems, in terms of speed, reliability and safety, has led to many proposals to automate various aspects of train operation. Controlling the movement of trains in a modern environment both in a train yard and on main train lines is a complex process. Collisions with other trains must be avoided and regulations in areas such as grade crossings must be complied with.
Trains or a maintenance crews must be coordinated by a dispatcher to occupy a portion of main line track between named locations (e.g., mile markers, switches, stations, or other points). In addition to specifying certain track sections, dispatchers must be able to coordinate trains and crews with respect to specifying speed limits, direction, time limits, and whether to clear the main line (e.g., by entering a secondary track such as a siding) and/or any other section of track (sidings, yards secondary track, etc.). Any errors in this process can lead to disastrous consequences.
Attempts to automate the above-described track coordination system include Centralized Traffic Control (CTC) systems which allow a dispatcher to control movement of trains by controlling track switches and wayside signals from a central dispatch office. More advanced systems include Automatic Train Control (ATC) systems where train location, speed and train control information are continually exchanged between a train cab and computerized wayside controllers in real time (in some systems, often referred to as cab signal systems, track rails are used to carry this information). The more advanced versions of CTC and ATC systems often employ GPS technology for accurate positioning information for speed, reliability and safety reasons.
Given the foregoing, one can conclude that the accuracy of any particular standalone GPS receiver (e.g., located on a train car), or collection of GPS receivers (e.g., several receivers working as part of a CTC or ATC system) is of concern. Any given GPS receiver can have an accuracy (i.e., can have errors in their positioning determination) ranging from 10 to 100 meters. The accuracy of a GPS receiver is affected by several different factors that can be categorized as either “natural” or “military.”
As for the natural category of errors, the position information provided by a GPS receiver is derived from determining the amount of time a signal takes to travel from the satellite to the receiver. This measurement is made possible by placing clocks in each of the satellites and the receivers. Errors in either the satellites' clocks or the receiver's clock alter this determination. Lack of stability or synchronicity among the clocks will result in an inaccurate measurement of signal travel time. When this is multiplied by the speed of electromagnetic radiation (i.e., the emitted L-band signal), an error in the apparent distance, will result.
A second natural source of error is in the value representing the propagation speed of electromagnetic radiation (i.e., the L-band radio signal). While the propagation speed of electromagnetic radiation is constant in a vacuum, it is retarded by passage through matter such as air in the atmosphere. The amount of speed alteration (i.e., delay) caused by the atmosphere will depend on the thickness of the air layer traversed, temperature, and a variety of other atmospheric conditions.
Apart from the “natural” category of errors in pseudorange determination and in determination of precise satellite positions, GPS also contains the capability to produce purposeful errors—known as selective availability (“SA”)—which can be introduced by the U.S. military. That is, in order to prevent the precision of GPS positioning from being used by the wrong persons, the military has the capability to introduce purposeful random errors into the clock signal broadcast by the GPS satellites. This has the effect of further degrading the accuracy of the pseudorange determinations and, hence, the accuracy of the coordinates determined for the GPS receiver.
A more detailed discussion of both the so-called “natural” and “military” categories of errors affecting the accuracy of GPS receivers can be found in U.S. Pat. No. 5,828,336 issued to Yunck, et al. which is incorporated herein by reference in its entirety.
A known method of improving the accuracy of a (standalone) GPS receiver's position determinations in spite of the above-mentioned category of errors is known as Differential GPS (DGPS). In this technique, one or more additional known locations are added to the GPS determination. Essentially, one or more ground stations in the general vicinity of a moving GPS receiver simultaneously receive the GPS signals and determine their own positions. Because the ground stations are stationary, any change in their determined position must be due to GPS error, either natural or military. The delta value between the ground station's true position and the position recently determined by GPS is broadcast so that mobile GPS receivers in the vicinity of the relevant ground station can use this correction to improve their own positioning solution.
Because mobile receivers in the vicinity of the ground station are receiving the same GPS satellites through essentially the same part of the atmosphere and at the same instant as the known ground station, these differential corrections are quite effective at overcoming the effects of the above-mentioned two categories of errors. Because the mobile GPS receiver is not at exactly the same coordinates as the ground station and the true programmed position of the ground station may not be perfect, however, the correction achieved by DGPS-type techniques is consequently not perfect as well. In addition, the DGPS receivers are more complex, and therefore more expensive, than ordinary GPS receivers.
In the transportation industry, it is important to know which path a vehicle has taken from among a plurality of possible fixed paths. In particular, in the railroad industry, it is important to know whether a train is on the correct track after passing a switch. If the switch is set at an incorrect position and the train has taken the wrong track, a collision may result. Ideally, track switches are set at the correct position so that a train will take the correct track and, in the event the switch is not correctly set, a train operator will stop the train before or shortly after passing the switch. However, human beings are imperfect and prone to mistakes. Thus, it would be desirable to have a system that can automatically determine whether a correct path has been taken. However, in many situations, alternate paths are often separated by a distance less than the accuracy of a GPS system receiver and are therefore not spaced far enough apart to permit an unambiguous determination as to which of two or more alternate paths have been taken by a vehicle.
Therefore, what is needed is a system, method and apparatus for determining whether a vehicle has taken a correct path when alternate paths are separated by a distance less than the accuracy of a positioning system receiver.